The circuit concept now known as the "current conveyor" was disclosed by K. C. Smith and A. Sedra in a paper entitled "The Current Conveyor: A New Circuit Building Block," proc. IEEE Vol. 56, pp. 1368-1369, Aug. 1968 and also in U.S. Pat. No. 3,582,689, issued June 1, 1971. Both of these documents are incorporated herein by reference.
The circuit building block embodying this concept was termed a first generation current conveyor or CCI. In the aforementioned documents, Smith and Sedra disclosed an elementary implementation which was not entirely satisfactory because of severe distortion and accuracy limitations due to base current errors and output impedance restrictions resulting from the "Early" voltage effect on the output transistor.
In 1969, Smith and Sedra disclosed a more accurate realization of the CCI in a paper entitled "A Few Simple Wide-band Current Measuring Device," IEEE Trans. Inst. and Meas., Vol. IM-18, pp. 125-128, June 1969, which is incorporated herein by reference. With the circuit in this disclosure, they attempted to compensate for the previously mentioned base current errors ant utilized a Wilson type of current mirror to improve the output impedance. In this specification, the term "Wilson current mirror" is used to donate a current mirror as disclosed by G. R. Wilson in a paper entitled "A Monolithic Junction FET-NPN Operational Amplifier," IEEE J. Solid State Cir. Vol. SC-3, pp. 380-387, December 1968, which is incorporated herein by reference. The Smith and Sedra circuit had a major practical limitation in that all transistors, including both NPN and PNP types, were required to match each other in current gain in order to realize the improved accuracy. This requirement cannot be met with any conventional type of integrated circuit processing where transistors of a like polarity match each other, but NPN and PNP current gains can differ widely. Additionally, although the Wilson mirror improved the output impedance, it was still only half that which could be realized with a pure cascode circuit.
In a paper entitled "A Second Generation Current Conveyor and its Applications," IEEE Trans. Circuit Theory, Vol. Ct-17, pp. 132-134, February 1970, which is incorporated herein by reference, K. C. Smith and A. Sedra disclosed another building block based upon the CCI concept but with different terminal characteristics. This was captioned the "second generation current conveyor" or CCII and was the subject of U.S. Pat. No. 3,619,798, issued Nov. 9, 1971, which is incorporated herein by reference. However, the accuracy of this implementation was limited by base current errors and output impedance in a manner similar to that previously described for the CCI.
In a current conveyor, current is conveyed between a low impedance input port and a high impedance output port. The current is ideally unaltered except for impedance level. A reference port is used to define the voltage level of the input port. In a first generation current conveyor or CCI, the reference port must support a current equal to the that which is conveyed between the input and output ports, whereas a second generation current conveyor or CCII is so configured that the reference port is high impedance. As such, the CCII reference port can perform its potential definition function while only supporting a very small (ideally zero) current.
In a paper entitled "Gyrator Implementation with Integrable Current Conveyors," IEEE J. Solid State Circuits, Vol. SC-6, pp. 396-399, December 1971, which is incorporated herein by reference, G. G. A. Black, R. T. Friedmann and A. S. Sedra disclosed a CCII using an operational amplifier and external transistors. They stressed the importance of using a cascode configuration to obtain high output impedance, but did not indicate how this could be done without introducing base current errors. In addition, their curcuit was subject to the frequency and transient response limitations of the operational amplifier.
In a paper entitled "A High Output Resistance Current Source," IEEE J. Solid-State Circuits, Vol. SC-9, pp. 192-194, August 1974, which is incorporated herein by reference, R. C. Jaeger discussed in detail the output impedance limitations of the Wilson and cascode current sources. As an improvement, he suggested an emitter referenced cascode current source, but the biasing of the bipolar realization of this idea was relatively complex, and the transient response was poor.
Between 1978 and 1980, a number of researchers disclosed implementations of current conveyors using operational amplifiers alone or in combination with operational transconductance amplifiers in an attempt to define a block that was suitable for integration. These disclosures included "A Current Conveyor Realization using Operational Amplifier, " Int. J. Electronics, Vol. 45, No. 3, 1978, by M. Sharif-Bakhtiar and P. Aronhime, "Novel Circuit IMplementation of Current Conveyors Using an O.A. and an O.T.A.," Electronics Letters, Vol. 16, No. 1, 1980, by R. Senani, and "Circuit Implementation of Current Conveyor," Electronics Letters, Vol. 16, 1980, by J. L. Huertas, and are incorporated herein by reference. However, these approaches were all limited by the performance of the operational amplifiers and required tight resistor matching to avoid seriously degraded performance.
In an effort to minimize the disadvantages of using operational amplifiers as building blocks for current conveyors, in a paper entitled "High-Performance Current Conveyor Implementation", Electronics Letters, Vol. 20, pp. 990-991, November 1984, which is incorporated herein by reference B. Wilson proposed to sense the output stage current in an operational amplifier and produce a current conveyor function with appropriately connected current mirrors. In particular, the positive and negative supply pins on a standard operational amplifier have been utilized to implement this idea. For a given operational amplifier, this approach improved the frequency response when compared with the operational amplifier circuits previously described, but transient response, accuracy, distortion, and output impedance remained problematic.
In a review of the state of the art of current conveyors in a paper entitled "Current Conveyors: A Review of the State of the Art," IEEE Circuit and Systems Mag., Vol. 3, No. 1, 1981, which is incorporated herein by reference, U. Kumar outlined the many applications of both the CCI and CCII circuits, but no new realizations of this circuit concept were presented. A number of other authors have similarly addressed only application related issued.
Several other papers in the literature in fact disclose variants on the current conveyor concept, although they are not identified as such. In a paper entitled "An Accurate Integrated Voltage to Current Converter, " IEEE J. Solid State Circuits, Vol. SC-10, December 1975, which is incorporated herein by reference, A. T. Van Zanten and J. H. Huijsing described a voltage-to-current converter. They made progress in the areas of accuracy and distortion, but the complexity of the feedback loop resulted in poor transient response. In a paper entitled "Report on the VCT," ETI Canada Magazine, February 1977, which is incorporated herein by reference, R. Harris outlined a similar concept called the "Voltage to Current Transactor" or VCT. As before, accuracy and distortion were improved, but output impedance was limited to that of the Wilson mirror. Another voltage-current converter was detailed by R. W. Barker and B. L. Hart in a paper entitled "A Novel Integrable Voltage-Current Converter", IEEE J. Solid State Cir., Vol SC-22, February 1987, which is incorporated herein by reference. Base current errors in this circuit, however, resulted in degraded accuracy, and the current mirror used had very poor transient response.
In a paper entitled "Wideband Translinear Current Converter", Electronics Letters, March 1984, which is incorporated herein by reference, A. Fabre disclosed a complementary configuration, using a Wilson current mirror and a six transistor output circuit. This arrangement, however, had first order base current errors, and although the configuration was apparently intended to give a higher output impedance, it would seem to give only one half of that of a pure cascode circuit.
Accordingly, although the current conveyor has attracted the interest of a considerable number of researchers over the last twenty years, it is evident that performance enhancements are still required to address the range of applications for this device.
An object of the present invention is to eliminate or at least amerliorate these various disadvantages outlined above.